Compositions are known and offered in different viscosity classes for different impression methods. For example, known compositions include kneadable and highly-viscous compositions which are for spoon application, and medium- and low-viscosity compositions which are preferably for spray application.
DE-B-17 45 810 provides impression compositions of polyether materials with aziridino end-groups.
DE-A1-37 41 575 and DE-A1-38 38 587 provide impression compositions based on polyether materials with alkenyl groups and polyorganosiloxane radicals, containing H--Si groups, which polymerize under the action of platinum catalysts.
EP-A2-0 173 085 provides impression compositions of polyether materials with acrylate and methacrylate groups which polymerize after irradiation with light of a suitable wavelength--initiated by the decomposition of a photo initiator.
The above polyether materials have a good ability to flow on hydrophilic oral surfaces and therefore a higher impression exactness can be obtained with these materials than with other known impression compositions, e.g., those based on conventional hydrophobic silicones. However, a disadvantage of these known polyether materials is that their water absorption is high when compared with silicone impression compositions. In this regard, the disinfection of impression compositions by means of aqueous disinfection baths is now indispensable and is frequently even repeated, in order to break the chain of infection. As a result, there has been an increased importance placed on achieving the smallest water absorption possible, as well as small swelling and therefore small changes in dimension.
Another disadvantage of the known polyether materials is their greater wetting angle compared with other known impression compositions, e.g., those based on hydrocolloids. The latter of these show a poor dimensional stability when compared with polyether materials. As a result of their high susceptibility towards swelling or shrinking, the hydrocolloids are not storage-stable, and impressions made therewith have to be cast immediately.
A larger wetting angle can lead to reduced flow and to the persistence of air inclusions when casting the impression with a plaster suspension. As a result plaster models of this type are unusable. The wetting angle is the angle which the edge of a water drop forms relative to the substrate surface (Walter Noll, Chemistry and Technology of Silicones, Academic Press, 1968, particularly pages 447-452).
Other known silicone impression compositions have the advantage of a relatively small water absorption, but their ability to flow on their wetting angles are not satisfactory. Attempts have therefore been made to confer a hydrophilic character upon these silicone compositions by incorporating a hydrophilizing agent and to reduce their wetting angle. Such compositions are for example described in EP-A1-0 480 238. A major disadvantage of the hydrophilized silicone compositions is, however, that the small wetting angle achieved through the addition of the hydrophilizing agent is to a very large extent lost even by a single disinfection in an aqueous disinfection bath.
The increased water absorption of hydrophilic silicones, which is many times that of the non-hydrophilized silicones, is particularly disadvantageous. Gribi, Quintessenz, Zahntech. 18, 1261-1274 (1992) describes in FIG. 7, page 1271 specimens of the same size which were exposed to flowing or standing water for 8 hours. After surface drying, the specimens were weighed. As a result it was found that the hydrophobic silicone (Coltene President Jet Lightbody) has a water absorption of 0.12%. The hydrophilized silicone (Coltene President Plus Jet Lightbody) absorbed 0.40% water, this corresponded to an increase around 3.3 times that of the hydrophobic silicone. A hydrophilized silicone with an extremely deep wetting angle absorbed 1.13% water, this corresponded to 9.4 times that of the hydrophobic silicone. A particular disadvantage of increased water absorption is the formation of hydrogen gas, which is triggered in the case of silicones by the crosslinker component. The formation of the hydrogen gas leads to bubbles in the model. Finally, the delayed setting of silicones hydro-philized to such an extent is also a disadvantage, since removal of the impression prior to the end of the hardening reaction can possibly deform the impression. Also, the setting thereof is inhibited by latex gloves, which for reasons of hygiene are indispensable in dental treatment.